Loop: an explicit enable, four named marks with grabbable caps, and the crossfade painted where it is actually heard

hasLoop becomes user-owned with the gestures as shortcuts onto it; no format change. START uses overlay/trace, not accent/primary, which is the waveform's own fill.

src/core/instrument/ui/loop_marks.cpp

@@ -0,0 +1,55 @@
+// loop_marks.cpp — see loop_marks.h. Pure value folds; no host types.
+
+#include "core/instrument/ui/loop_marks.h"
+
+#include "core/instrument/engine/loop/loop_span.h"  // defaultLoopBounds
+
+namespace reasampler::instrument::ui {
+
+namespace {
+
+// The engine's own acceptance test, restated over the editor's two integers: resolveLoop
+// refuses an inverted, empty or out-of-range span rather than repairing it, so a span it would
+// refuse is one the handles must be re-parked out of.
+bool spanUsable(std::int64_t loopStart, std::int64_t loopEnd, std::int64_t frameCount) {
+    return loopStart >= 0 && loopEnd > loopStart && loopEnd <= frameCount;
+}
+
+}  // namespace
+
+LoopMarks resolveLoopMarks(const StoredLoop& stored, std::int64_t frameCount) {
+    LoopMarks m;
+    if (stored.override_) {
+        m.hasLoop = stored.override_->hasLoop;
+        m.loopStart = stored.override_->start;
+        m.loopEnd = stored.override_->end;
+    } else if (stored.intrinsic && stored.intrinsic->hasLoop) {
+        m.hasLoop = true;
+        m.loopStart = stored.intrinsic->start;
+        m.loopEnd = stored.intrinsic->end;
+    }
+    if (stored.startPoint) m.start = *stored.startPoint;
+    m.crossfade = stored.crossfade > 0 ? stored.crossfade : 0;
+
+    if (!spanUsable(m.loopStart, m.loopEnd, frameCount)) {
+        m.hasLoop = false;
+        m.parked = true;
+        const engine::loop::LoopBounds d = engine::loop::defaultLoopBounds(frameCount);
+        m.loopStart = d.start;
+        m.loopEnd = d.end;
+    }
+    return m;
+}
+
+LoopWrite applyLoopMarks(const LoopMarks& m) {
+    LoopWrite w;
+    const bool spanAlive = m.loopEnd > m.loopStart;
+    w.loop.hasLoop = m.hasLoop && spanAlive;
+    w.loop.start = m.loopStart;
+    w.loop.end = m.loopEnd;
+    w.crossfade = (spanAlive && m.crossfade > 0) ? m.crossfade : 0;
+    w.start = m.start;
+    return w;
+}
+
+}  // namespace reasampler::instrument::ui